When paramecia are placed in dilute homologous antisera, they become immobilized and eventually die. This reaction, the immobilization reaction, is the result of a specific interaction between homologous antibodies and the immolulization antigen (i-antigen) located on the surface of the cell. The i-antigen is distributed over 90 percent of the cell membrane. While a clonal population of cells has been shown to express only one antigen (serotype) at any given time, individual cells are capable of expressing upwards of twelve different antigens giving rise to the phenomenon of antigenic variation. A systematic phenomenon of antigenic variation has now been shown to exist in several species of protozoa including both the free-swimming forms such as paramecia and the parasitic forms, such as trypanosomes.